The Moon Is 100 Million Years Younger Than We Thought

The moon is quite a bit younger than scientists had previously
believed, new research suggests.

The leading theory of how the moon formed holds that it was created when
a mysterious planet — one the size of Mars or larger — slammed
into Earth about 4.56 billion years ago, just after the solar
system came together. But new analyses of lunar rocks suggest
that the moon, which likely coalesced from the debris blasted into space
by this monster impact, is actually between 4.4 billion and 4.45
billion years old.

The finding, which would make the moon 100 million years younger
than previously thought, could reshape scientists' understanding
of the early Earth as well as its natural satellite, researchers
said. [The Moon: 10 Surprising Lunar Facts]

"There are several important implications of this late moon
formation that have not yet been worked out," Richard Carlson, of
the Carnegie Institution for Science in Washington, D.C., said in
a statement.

"For example, if the Earth was already differentiated prior to
the giant impact, would the impact have blown
off the primordial atmosphere that formed from this earlier epoch
of Earth history?" added Carlson, who is presenting the new
results Monday (Sept. 23) in London at a meeting organized by the
Royal Society called "Origin of the Moon."

Scientists know the solar system's age (4.568 billion years)
quite well. And they can pin down the formation times of
relatively small bodies such as asteroids precisely, too, by
noting when these objects underwent extensive melting — a
consequence, in part, of the heat generated by the collision and
fusion of these objects' building-block "planetesimals."

For example, analysis of meteorites that came from the asteroid
Vesta and eventually rained down on Earth reveals that the
330-mile-wide (530 kilometers) space rock is 4.565 billion years
old. Vesta cooled relatively quickly and is too small to have
retained enough internal heat to drive further melting or
volcanism, Carlson explained.

But it's tougher to nail down the age of larger solar-system
bodies, he said.

"Ask the same question of the Earth or moon, and you don't get a
very precise answer," Carlson said. "Earth likely took longer to
grow to full size compared to a small asteroid like Vesta, and
every step in its growth tends to erase, or at least cloud, the
memory of earlier events."

Scientists keep getting better and better estimates, however, as
they refine their techniques and technology. And those estimates
are pushing the moon's formation date farther forward in time.

The moon is thought to have harbored a global
ocean of molten rock shortly after its dramatic formation.
Currently, the most precisely determined age for the lunar rocks
that arose from that ocean is 4.360 billion years, the
researchers said.

And here on Earth, scientists have found signs in several
locations of a major melting event that occurred around 4.45
billion years ago. So, evidence is building that the catastrophic
collision that formed the moon and reshaped Earth occurred around
that time, rather than 100 million years or so before, the
researchers said.